Throw when reflecting on elements not covered by a `MirrorsUsed` annotation.

sdk/lib/_internal/compiler/implementation/mirrors_used.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors_used.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors_used.dart b/sdk/lib/_internal/compiler/implementation/mirrors_used.dart
index 144f3950d2abbe5d4f3dff27f6c412c95e5639a5..98707c653cd0d4e1f7833997f182bf2c8c3fb43c 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors_used.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors_used.dart
@@ -562,6 +562,10 @@ class MirrorUsageBuilder {
   Element findLocalMemberIn(Element element, SourceString name) {
     if (element is ScopeContainerElement) {
       ScopeContainerElement scope = element;
+      if (element.isClass()) {
+        ClassElement cls = element;
+        cls.ensureResolved(compiler);
+      }
       return scope.localLookup(name);
     }
     return null;